One of the lake's most important water-quality tools is invisible, sinks to the bottom, and works by chemistry.

When residents complain that a lake looks green and soupy in late summer, the culprit is usually phosphorus, a nutrient that feeds algae. It is one of the least visible problems on the Chain of Lakes and one of the most consequential, and the main tool used against it is itself invisible: it sinks to the bottom and works by chemistry, out of sight of the person standing on the dock.
Decades of stormwater carried phosphorus into the Chain of Lakes, where it settled into the sediment and kept feeding blooms long after the original pollution stopped. This is the frustrating part of lake chemistry: even when a city cleans up its runoff, the nutrient already banked in the mud keeps cycling back into the water, a condition scientists call internal loading.
Among the tools used at Cedar and Isles to attack that legacy were alum treatments, applied directly to the lakes to bind phosphorus and pull it out of circulation. Alongside grit chambers, constructed wetlands and significant shoreline restoration, the treatments were part of the coordinated effort that the Park Board and its partners have described as the nation's largest urban lake restoration.
Alum, or aluminum sulfate, reacts in the water to form a floc that captures phosphorus and carries it to the lake bottom, where it stays locked away from the algae that would otherwise feast on it. The visible result, over time, is clearer water and fewer severe blooms, the difference between a lake you want to swim in and one you would rather not.
It is a targeted intervention rather than a blunt one. A treatment is timed and dosed to the conditions of a specific lake, and the effect can last for years before the banked phosphorus rebuilds enough to warrant another round. That durability is what makes alum worth the cost on a chain this heavily used.
It is not, however, a permanent fix. Phosphorus keeps arriving with every storm, which is why the chemical treatments are paired with the slower structural work of wetlands and restored banks that intercept the nutrient before it ever reaches the lake. Treat the symptom and ignore the source, and the bloom comes back.
Alum is powerful precisely because it is paired. On its own it would be a temporary fix, binding the phosphorus already in the lake while doing nothing about the phosphorus still arriving. Combined with the grit chambers and constructed wetlands that intercept runoff and the restored banks that slow it, the treatment becomes one move in a coordinated strategy that the Park Board and its partners have built up over decades on the chain.

Hennepin County is expected to bring its final design for rebuilding Lyndale Avenue South to the Minneapolis City Council this month, after a June 1 public meeting where Uptown business owners and cyclists clashed over a plan that adds a bikeway and cuts about a quarter of on-street parking.

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That strategy is why the lakes earned their place in what has been called the nation's largest urban lake restoration. For the swimmer or paddler who never thinks about water chemistry, the relevant result is simple and earned: clearer water in late summer, fewer of the green, soupy weeks that once drove people off the lake, and a clarity maintained by a deliberate, ongoing program rather than luck.
It helps to think of the lake as having a memory. Decades of stormwater banked phosphorus in the sediment, and that stored nutrient keeps feeding blooms even after the inflow is cleaned up, a process scientists call internal loading. Alum is, in effect, a way to wipe part of that memory clean, locking the banked phosphorus to the lakebed so it can no longer fuel the next summer's algae.
For the person on the dock, the takeaway is reassuring and sobering at once. The clearer water is real and earned, but it is maintained, not given. Behind a clean late-summer swim is a chemistry experiment the Park Board has been running, quietly, for years, and intends to keep running.
The Lowry Hill Neighborhood Association board meets the first Tuesday of each month, 7 to 9 p.m., at the Searle Mansion, 1915 Logan Ave. S., where parks requests, traffic concerns and land-use notices get aired.

For the first time in years, the Hennepin Avenue corridor through Uptown heads into summer without an active construction zone, the rebuilt street now served by the METRO E Line that began carrying riders in December.